Zirconia sols, or the finely divided crystalline particles obtained from zirconia sols are useful in a variety of applications. These can range from ceramics in which the zirconia serves to bind components together to plastic materials in which zirconia serves a similar function. Likewise, zirconia sols can be used to bind organic acids or other organic compounds such as dyes.
It also is known in the prior art that colloidal zirconium oxide sols can be stabilized by the incorporation of other components such as yttrium oxide. Ceramic bodies made from such stabilized zirconium oxide sols containing, for example, about 5 to 6% by weight of Y.sub.2 O.sub.3 exhibit high toughness and strength which is not obtained by traditional oxide ceramics. In many applications, the preparation and use of finely divided oxide powders of zirconium is particularly desirable since, in general, the mechanical properties improve with decreasing particle sizes. With the rapid advances in the electronics field, ultra-pure zirconia is desired for use in the production of electronic devices. Thus, there is a continuing need for stronger, more detailed ceramics for high performance electronic devices, and a great deal of effort has been devoted to developing improved zirconia for use as a starting material for the production of such products.
The patent literature contains many suggestions for preparing zirconium oxide having improved properties for various uses. U.S. Pat. No. 2,438,230 describes a method for producing aqueous colloidal solutions (sols) of hydrous oxides of various metals such as aluminum, iron, cobalt, manganese, nickel or other divalent or trivalent metals. The sols are prepared by passing dilute aqueous solutions of the salts capable of forming insoluble hydrous oxides through a suitable anion-exchange material. This patent, however, does not suggest the use of zirconium as one of the metals, the use of an oxy salt such as zirconium oxychloride or the formation of essentially uniform fine particles of about 0.5 micron diameter.
U.S. Pat. Nos. 2,546,953 and 2,759,793 discuss the use of ion-exchange resins, preferably cationic-exchange resins, to separate zirconium from hafnium. The resin is then eluted with hydrochloric acid or sulfuric acid, and the eluate containing the higher concentration of zirconium is separated and the zirconium is recovered.
U.S. Pat. No. 2,984,576 describes the use, as refractories, of zirconium oxide sols having particles in the range of 0.005 to 0.2 microns in diameter. The sols are prepared from a basic aqueous solution of a zirconium salt such as zirconium oxychloride or other salt such as zirconium nitrate. The salt is hydrolyzed under pressure at a temperature of 120.degree.-300.degree. C. for 1-2 hours followed by removal of excess acid. The sol, having a final pH of between 2 and 6 can be concentrated to a desirable concentration, and the salt can be converted to a dry, dispersible powder if desired.
U.S. Pat. No. 3,032,388 describes a process for the production of zirconium oxide from impure zirconium chloride which also contains hafnium which is naturally associated with the zirconium. The impure zirconium chloride is subjected to liquid-liquid extraction to separate the zirconium from the hafnium. The hafnium and zirconium solutions are separately processed to recover the two constituents in pure form. U.S. Pat. No. 3,157,601 describes the preparation of metal sols with two oxide constituents which can be easily hydrolyzed. Such sols include the salts of zirconium, titanium, aluminum, chromium, etc., and the sols are prepared using urea added to the mixture of metal chlorides (e.g., uranous chloride and thorium chloride) followed by passing the sol through an ion-exchange resin. The process described in the '601 patent depends upon the use of two salts which hydrolyze at a different pH thereby resulting in particles composed of two oxides wherein the oxide which hydrolyzes at the higher pH coats the oxide which hydrolyzes at a lower pH.
U.S. Pat. No. 3,282,857 describes the preparation of aquasols of crystalline zirconia and/or hafnia. The sols are prepared by (1) electrodialysis of an aqueous solution of a basic zirconium or hafnium salt followed by (2) autoclaving of the zirconia or hafnia sol at a suitable temperature pressure. During the autoclaving, the pH drops, and electrical conductivity rises as anion is released by the sol particles to the surrounding medium. Alternatively, the first step may involve passing the solution of a basic zirconium or hafnium salt through an anion exchange resin. The product sol resulting from the second step (i.e., autoclaving) may be deionized by stirring it with a weak anion-exchange resin such as Amberlite IR-45. Zirconium oxide and hafnium oxide sols also are described in U.S. Pat. No. 3,442,817, and the sols are prepared from zirconyl or hafnyl acetate by heating to about 175.degree.-400.degree. C. which forms colloidal particles which are then treated with a cationic exchange resin to remove free ions of zirconium or hafnium (deionization).
U.S. Pat. No. 3,676,362 describes the preparation of sols of antimony oxide, tin oxide, titanium oxide and zirconium oxide in organic polar liquids. Such sols are prepared by reacting a soluble salt of the metal with water and ammonia in a solution of the liquid. The reaction produces metal oxide in colloidal dispersion and an insoluble ammonium salt which precipitates and is removed by filtration. Sols in liquids which are solvents for organic polymers are used to incorporate metal oxides into polymers to provide flame-resistance and other desirable properties. U.S. Pat. No. 3,850,835 describes a method for producing zirconium hydrous oxide by mixing a zirconium compound such as zirconium oxychloride with a reagent such as a water-soluble phosphate.
U.S. Pat. No. 4,588,576 describes a process for the production of a micro-crystalline metal oxide having an average particle size less than about 1000 Angstroms. Zirconium oxide particles of about 0.1 micron diameter or smaller are described. These particles are spherical in structure and can contain yttria as a minor component. The particles are prepared from a solution containing a soluble zirconium metalorganic compound which is subjected to ultrasonic wave energy to form a gel. The solvent medium is removed from the gelled solution to provide a mass of precursor solids, and the precursor solids are pyrolyzed in the presence of molecular oxygen to form a metal oxide composition.
Various zirconia porcelains and ceramics stabilized with other metal oxides such as cerium oxide, yttrium oxide, calcia, magnesia, etc. are described in a number of patents including, for example, U.S. Pat. Nos. 4,067,745; 4,279,655; 4,525,464; 4,565,792; 4,610,967; 4,659,680; 4,664,894; and 4,666,467.